Vendors, purchasers, consumers, and transporters of hazardous materials demand that their containers meet increasingly difficult tests. Typically, the tests are those defined by associations or regulatory bodies; a good example is the United Nations Specifications for the Transportation of Hazardous Materials, which emphasize particularly the internal pressure effects of volatile materials subjected to variable temperatures over long periods of time, and/or a drop test which may cause significant abrupt increases in pressure in a filled container. Internal forces caused by vapor pressure or severe impact may rupture, dislodge or distort the seal between the container and the lid in certain known and predictable ways.
Typically a sealed container filled with a volatile, high vapor pressure material, when subjected to increase in ambient temperature or when stored in a tropical climate or transported in closed vehicles will experience an increase in internal pressure and tend to cause a doming effect or a lifting of the center of the lid; this is especially so in the case of many containers of current design, which have a peripheral gasket around the top rim of the container itself, kept in place by contact inside a circumferential recess in vertical relation to the rim of the lid. While the conventional gasket placement achieves a good seal in normal usage, it is in a relatively weak and flexible portion of the assembly when the container is subjected to distortion by impact and internal pressure. The doming effect will distort the outer contour of the lid and cause a separation of the gasket and the lid, resulting in an opening and spillage or forceful spewing of the hazardous material. This effect has been partially overcome in the past through the use of a peripheral inwardly directed ledge on the lid and a complementary peripheral chime on the container; the lid including the gasket is snap-fitted onto the container so the ledge will make it difficult to lift the lid. Frequently, however, the sudden distortion of impact under the drop test and/or a time factored internal (vapor) pressure test will cause a separation and/or loosening of the lid.
It should also be noted by way of background that some of the limitations in the design of construction of the container-lid interface are due to the difficulty of removing a newly molded thermoplastic lid from a mold. Most especially, newly molded lids which have peripheral inwardly directed ledges made to contact the underside of peripheral chimes on the containers have been removed from the molds by stripper rings which stretch the thermoplastic outwardly so as to clear the larger dimensions of the mold. The dimensions of the inwardly directed ledges have been limited by the limits of elasticity of the thermoplastic. A mold construction for readily removing the perimeters of thermoplastic lids from mold cores which include peripheral protrusions into the lids is disclosed in my U.S. Pat. No. 4,777,004.
Accordingly, in designing a new thermoplastic lid which will cooperate with the container body in such a way as to pass the more stringent drop tests and other tests, one must take into consideration both the way the lid is made and the manner in which stresses are distributed in the filled container when it is subjected to the various tests for impact and internal pressure. As may be seen below, the seal effected by the gasket may be employed in such a way that it is not weakened by a doming effect of the lid.